Normally we say "sofek d'rabbanan l'kulo" and will be lenient in a case of a doubt that is only an issurd'rabbanan. I have heard that there are poskim who say this doesn't apply in the case when there is a "sofek d'rabbanan" by the tevillah (immersion) of a Niddah (for example a question of a chatzitzah) since this is a "davar sheyesh lo matirim": she would be able to easily go back to the mikveh and tovel again. Therefore we don't say "sofek d'rabbanan l'kulo". Who, is anyone, says this? Any sources? Any poskim that apply this l'maseh (perhaps found in teshuva seforim or other halacha seforim)?

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First let me say that the laws of Niddah in general and the laws of chatzitzah in particular are very complex. In general the rule is that if she has some valid reason to suspect that she handled something that was an actual chatzitzah she needs to immerse again. See Shulhan Arukh Y"D 198:41 and 199:13 for examples.
Now there are cases of certain items that the mikvah lady would normally say are a chatzitzah, that if she immersed in them the immersion would still be valid say clothes for example Y"D 198:46, and see specifically Rav Moshe Feinstein's teshuva regarding women swimming in the ocean counting as mikvah(Y"D II 96 I believe).
In other cases we may say that the immersion was a sofek, but if she has been with her husband she need not immerse again(see Lechem V'Simla on Y"D 198:41).
Essentially yes there are situations, however there are many factors involved, and they cannot all be adequately discussed in this sort of format.

Most people realize this. The question however was specific to such an idea or concept if it exists or is found in poskim, seforim, etc
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YehoshuaApr 3 '13 at 20:12

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It does. Hence I noted the Lechem V'Simla and the Iggrot Moshe. Both deal with cases of a sofek chatzitzah and under what circumstances we can be lenient in the given situation. If you are looking for a comprehensive list of responsa regarding those leniencies... well that is another matter.
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Rabbi Michael TzadokApr 3 '13 at 20:31